An optical fiber is known to exhibit a birefringence due to the refractive-index difference along two orthogonal axes (fiber birefringence axes). This difference arises from both the practical impossibility of manufacturing a perfectly circular fiber and from imperfections caused by the manufacturing process.
Birefringence causes a variation in the state of polarization of a radiation propagating along the fiber. Such a phenomenon is characterized by a well-defined periodicity. The distance between two subsequent points of the fiber where the propagating radiation presents the same state of polarization is called the beat length, and it is the distance at which the difference of phase .phi..sub.x,y between the components, along the two birefringence axes, of the electrical field associated with the radiation, has a value 2.pi..
As known, such a phase difference depends on the difference between the refractive indice along the two axes according to relation ##EQU1## where .lambda. is the radiation wavelength, Z is the distance covered within the fiber and n.sub.x, n.sub.y are the two refractive indices.
Polarization maintenance is fundamental when using single-mode optical fibers as sensors. Highly-birefringent fibers are used in these applications, these fibers are obtained by imposing the birefringence during manufacturing. A strong anisotropy derives from non-circular core shape and/or from permanent mechanical stresses induced on the core itself. Such fibers have a beat length of the order of some millimeters. Accurate knowledge of the beat length is indispensable for correctly dimensioning fiber lengths to be used in such applications.
Various methods for measuring polarization beat length in highly-birefringent optical fibers are known. For instance, "Precision measurement of modal birefringence of highly-birefringent fibres by periodic lateral force" by K. Takada, J. Noda and R. Ulrich, Applied Optics, Vol. 24, No. 24, 15 Dec. 1985, pages 4387 ff., describes a method where a transverse force is applied to the fiber according to one of the birefringence axes, this force is modulated (varied at audio frequency by a loudspeaker, the force application point and the modulating means are displaced along the fiber and the ratio between the components along the two birefringence axes, of the intensity of a radiation injected into the fibre is measured. The beat length is the length of the fiber over the displacement at which this ratio recovers the same value.
This method has a number of disadvantages: measurement conditions are to be such that only one of the fundamental modes is excited and coupling coefficient of the two modes is to be very small; since the signal to be measured is proportional to this coefficient, noises of even low intensity can seriously affect the measurement precision; fiber birefringence axes must be previously identified, and this lengthens measurement time; and the presence, in one device displaceable along the fiber, of means which apply a force to modify the state of polarization and means for periodically modifying this force to generate a reference for the measurement instruments makes it difficult to accurately control force intensity and direction, and this also is detrimental to measurement precision.